The present invention relates to high intensity discharge lighting systems, and to high intensity discharge lamps and controls therefor.
High intensity discharge lighting systems comprising a high intensity discharge lamp and a control for regulating the electrical power to the lamp are known. In this specification, references to xe2x80x9chigh intensity discharge lampsxe2x80x9d are to lamps having a sealed envelope containing at least two electrodes for an electrical discharge, and are arranged to be used for lighting when an arc is established across the electrodes. Such lamps have a high impedance before they are lit, and a low impedance while they are lit. Before the lamp is lit, it is necessary to apply a high voltage (typically 2-5 kV) across the lamp to start the lamp to conduct electricity. High intensity discharge lamps are characterised by a short arc length, typically less than 20 mm for a 70 watt lamp, and typically have a high internal pressure when hot. The envelope is filled with fill materials that may not be fully evaporated and hence have a low pressure when the lamp is cold and before the lamp has started conducting. However, when the lamp is operating and is hot, the said fill materials have a high pressure. High intensity discharge lamps are further characterised in that as a result of this increase in pressure of the fill material an ignition voltage required to start such lamps may increase sharply as the lamp becomes hot. For example, a lamp with a cold ignition voltage of 2,000 volts may when hot require an ignition voltage of 30,000 volts to restart the lamp. Additional electrodes may be provided in such lamps for particular applications to meet particular operating requirements.
Such known controls may comprise an electro-magnetic inductance to regulate the power, and a capacitor and switch arrangement to generate the high starting voltage. Such electromagnetic controls provide an electrical output to the lamp at the same frequency as the electrical supply to the control. Alternatively, electronic controls are known, where an electronic circuit is arranged to provide both the regulation and generate the high starting voltage. Such electronic controls normally provide an electrical output to the lamp at a higher frequency than that of the electrical supply to the control. Typical electronic controls for operating a high intensity discharge lamp produce a square wave voltage output at a frequency of up to 400 Hz with an electrical supply having a sinusoidal waveform and a frequency of 50 Hz or 60 Hz. These are hereinafter referred to as xe2x80x9csquare wavexe2x80x9d technology controls.
The arrangement for producing a high voltage for starting or igniting the lamp, being known hereinafter as an xe2x80x9cignitorxe2x80x9d, and the means for regulating the power when the lamp is operating in the lit state to provide a desired operating power for the lamp being known hereinafter as a xe2x80x9cballastxe2x80x9d.
In electronic controls known means to generate high voltage includes resonant circuits and suddenly discharged capacitor circuits. Known electronic controls having a self oscillating circuit operate at a frequency determined by the resonance of power handling components in the control circuit. A benefit of these self oscillating circuits is simplicity and low cost, however a disadvantage is that it is difficult to vary the operating frequency of such a control circuit as the operating frequency is determined solely by the values of fixed components, the values of which are determined by the power of the circuit it is arranged to control. Also known are electronic controls where the operating frequency is determined solely by a frequency generator such that the operating frequency can be arranged to be independent of the characteristics of power handling components in the circuit.
The electronic controls employed to date have, as a result of their complexity, a disadvantage of cost that has prevented their widespread use.
One of the reasons for the complex design of square wave technology controls, (which operate lamps at relatively low frequencies 50-400 Hz for example), is that discharge lamps exhibit undesirable instabilities when operated in the frequency range of 1 kHz-300 kHz depending on lamp type and geometry. Consequently, elaborate electronic topologies are required to generate low frequencies with power levels and control characteristics suited to discharge lamps.
Should the operating frequency (or some harmonic or sub harmonic of the operating frequency) be such as to excite standing waves of pressure within a lamp then undesirable movement or even extinction of the arc can occur. This can be damaging to the lamp since arc movement can cause the arc to impinge upon an inner surface of the envelope forming burner walls with consequent lamp failure. At the very least, these movements of the arc spoil the quality of illumination obtained.
The above mentioned instability and standing waves of pressure are manifestations of a phenomenon known as xe2x80x9cacoustic resonancexe2x80x9d. Acoustic resonance arises as a result of pressure variations in the lamp caused by the operating frequency or some harmonic or sub harmonic of the operating frequency. A lamp has an acoustic resonant frequency range that is the range of frequencies which will excite acoustic resonance within the lamp. Hence a particular lamp would be likely to exhibit acoustic resonance when operated with a power input frequency within the acoustic resonant frequency range.
For a particular lamp, the acoustic resonance conditions during the starting of the lamp will be different to those when the lamp is operating in a stable lit condition. Since the starting of the lamp is a transient phase of operation lasting a very short time interval such acoustic resonance phenomena that might otherwise occur during this transient phase do not normally have time to become established. Hence the acoustic resonant frequency range is defined with reference only to the conditions when the lamp is operating in a stable lit condition.
A high intensity discharge lighting system having a control and at least two high intensity discharge lamps is described in U.S. Pat. No. 5,986,412 to Collins. FIG. 1 of Collins"" Patent shows that the operation of the two lamps 12 and 14 is by means of an electromagnetic control, referred to as ballast circuit 10 which has a shared portion of the circuit comprising principally transformer 16, and two ignitor pulse circuits 30 and 50 for starting lamps 12 and 14 respectively. In operation lamp 12 must start before lamp 14 in order to conduct the electrical power necessary to operate the second ignitor pulse circuit 50. A disadvantage of the Collins system is that it is necessary to duplicate the ignitor circuit.
U.S. Pat. No. 5,982,109 to Konopka shows in his FIG. 4 two lamps 10 and 20 connected to an electronic control 120, and in FIG. 6 two lamps 10 and 20 connected to a control 160. In each case the lamps are connected in parallel current paths, and the only shared part of the control is the inverter 200, each lamp having its own output circuit 300, 500 and 400, 600 inductor 310, 510 and 410, 610 and other ignitor components. The Konopka arrangement has similar disadvantages to the Collins system in that it requires considerable duplication of expensive components.
U.S. Pat. No. 5,900,701 to Guhilot in FIG. 4C shows a plurality of lamps 16 connected in parallel across a secondary winding 111 of an inverter transformer 115. For each lamp so connected it is necessary to duplicate a ballast filter comprising capacitor 112 and inductor 113. A reason that it is necessary to duplicate the ballast filter components is to ensure stable and safe operation of each lamp, since being in parallel if one lamp failed to start all the output power from the transformer would pass through the single lit lamp. Hence, as in the previous examples duplication of expensive components is required.
U.S. Pat. Nos. 5,828,185 and 5,998,939 to Philips Electronics in FIG. 8 shows two light emitting elements, a first and a second discharge devices 3 connected electrically in series within a common outer bulb (column 12, line 20). A reason for combining two discharge devices in this patent is to overcome a disadvantage of the patent in that to achieve operation of the lamps below a lamp resonant frequency that would excite acoustic resonance, the size of the discharge devices must be such that the lowest lamp resonant frequency must be higher than the output frequency of the ballast. By limiting the physical size of the lamps, the maximum obtainable light output is also limited, and in Philips the power is limited to 20W. This is a severe restriction since the most commonly used high intensity discharge lamps are in the range of 35W to 150W. Further there are significant manufacturing difficulties to be overcome in the manufacture of small high intensity discharge lamps, as generally the fill within the discharge device must be at a much greater pressure in order to achieve suitable electrical discharge characteristics. The use of multiple discharge devices within one common outer bulb is also disadvantageous in that there is no longer a single light emitting point, and it may not be possible to achieve a desired focused lighting effect with a reflector.
None of the above patents disclose or teach the use of a control where there is no duplication of control components to enable the operation of a plurality of commercially available high intensity discharge lamps from the one control. Neither do any of the above patents disclose or teach means to ensure balanced operation of two lamps in series, or means to monitor the operation of lamps in series to enhance the safety of the lighting system.
According to one aspect of the invention there is provided a lighting system comprising at least:
(a) a plurality of high intensity discharge lamps, each of the high intensity discharge lamps comprising a sealed envelope containing at least a first and a second electrode for an electrical discharge,
(b) the lamps having an acoustic resonant frequency range,
(c) an electronic control having a power input, an alternating current power output regulator, and an alternating power output having an output frequency arranged to be variable within an output frequency range, the output having a first and a second output line,
(d) the lamps being connected in series with each other so that the first line is connected to the first electrode of the first of the lamps, the second electrode of the first of the lamps is connected to the first electrode of the next lamp in the series, the second electrode of the final lamp in the series being connected to the second output line, and
(e) the output frequency range being above the acoustic resonant frequency range.
A benefit of this lighting system is that it is not necessary to duplicate any components within the control, and hence a cost saving may be obtained while using commercially available standard high intensity discharge lamps.
A benefit of the output frequency range being above the acoustic resonant frequency range is that when the lamp is operating in a stable lit condition, the output frequency may be above an upper limit frequency which will excite acoustic resonance at the upper limit frequency. This improves the stability of the lamp operation.
Preferably, the control comprises a ballast and an ignitor, the ignitor having an ignition capacitance in a parallel current path with the lamps, the ignition capacitance being arranged in resonant circuit having a fundamental resonant frequency.
In an embodiment of the invention, preferably the control is provided with a switch to disconnect the ignition capacitor when the lamps are lit.
A benefit of disconnecting the ignition capacitor is that the efficiency of the control may be improved. A further benefit being that the ignition capacitor may be arranged to be resonant at a fundamental resonant frequency, and hence the ignition capacitance may be relatively large, providing a high energy high voltage starting or ignition condition.
In an alternative embodiment, preferably the ignition capacitance is arranged such that resonance occurs at a frequency above that used to provide a high voltage for igniting the lamps. More preferably, the ignition capacitance is arranged to resonate at the fundamental frequency when a particular output frequency within the output frequency range is at a third harmonic of the fundamental frequency.
A benefit of the resonance occurring at the third harmonic is that losses arising from a current drawn by the capacitor during the operation of the lamp when lit are reduced. A further benefit of the ignition capacitance being arranged to resonate when a particular output frequency is at a third harmonic of the fundamental resonant frequency and within the output frequency range is that a value of the ignition capacitance is sufficiently small that the capacitance does not require disconnection when the lamps are lit and that a current drawn during an ignition resonance is also reduced, and hence switching components in the control may be smaller.
Preferably, in an embodiment of the invention, a capacitor is connected in a parallel path separately with each lamp. Preferably, said capacitor is a portion of the ignition capacitance.
Preferably, in an alternative embodiment of the invention, a resistor is connected in parallel with each lamp.
A benefit of connecting a capacitor or a resistor in parallel with each lamp is that any imbalance between the power used by each lamp may be minimised. A further benefit is that safety may be improved since a failure mode caused by running a lamp above its intended power rating is that of explosion.
Preferably, the control is arranged to monitor a mid-point voltage level between an adjacent pair of lamps.
A benefit of this is that the mid-point voltage level measured at a point between the two lamps may be used to indicate the relative power consumption of each lamp.
Preferably, the control is arranged to power down when a measured value of the mid-point voltage level is outside a permissible range of values.
A benefit of the control being arranged to power down is that the control may be arranged to stop operating when a lamp is about to fail.
Preferably, the control has indicator means arranged to indicate which lamp has an arc voltage outside of a permissible range of arc voltage.
Preferably, the control has indicator means arranged to indicate which lamp has the lower arc voltage.
Preferably, the control has indicator means arranged to indicate which lamp has the higher arc voltage.
A benefit of this is that arc voltage may be used as an indicator of incipient lamp failure, or of a lamp that has failed. By way of example, a lamp that has failed to light will have the whole output voltage across it, while a lamp which has lost fill pressure will probably have a lower arc voltage. A further benefit is that should the control also be arranged to power down in the event of incipient lamp failure, indicator means will enable the faulty bulb to be replaced without requiring further investigations.
Preferably, the regulated alternating current power output for powering the lamps has an output frequency greater than a frequency of a power supply to the ballast.
Preferably, the control has a direct current to alternating current converter producing the regulated alternating current power output for powering the lamps, the control having an output frequency range above 300 kHz.
More preferably, the alternating current output of the control for powering the lamps has an output frequency range above 400 kHz.
A benefit of a frequency range above 300 kHz is that operation of the lamp may be further improved, and yet further improvements may be obtained by operation above 400 kHz.
Preferably, the output frequency range is entirely above the upper limit frequency of the acoustic resonant frequency range of the lamps.
A benefit of powering the lamps at a frequency above the upper limit frequency of the acoustic resonant frequency range of the lamps is that a more stable operation and a better quality of illumination is obtained. A further benefit of operating above the upper limit frequency of the acoustic resonant frequency range is that this does not place a constraint on the maximum size of the sealed envelope or burner.
Preferably, the alternating current output of the control for powering the lamps has a sinusoidal waveform.
A benefit of a sinusoidal waveform output to the lamps is that problems arising from harmonics present when a square wave waveform output is used may be avoided.
A further benefit of operation at high frequency is that the efficiency of the control may be further improved.